Alternating Current (AC) and Direct Current (DC) electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including voltage, current, capacitance, impedance, resistance etc. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment from high voltage electrical transmission lines operating at hundreds of kiloVolts (kV) and kiloAmps (kA) to industrial/medical/residential electrical and lighting, typically 400V/240V/100V and 30/15 A, to a wide variety of industrial/scientific/medical/consumer electrical and electronic devices.
Within a variety of applications and test equipment comparator bridges, e.g. AC comparator bridges and DC comparator bridges, are employed to provide the required dynamic range, accuracy, and flexibility. Such bridge configurations remove many of the issues associated with achieving making measurements at accuracies of a part, or few parts, per million such as insensitivity to lead resistances, excellent ratio linearity, excellent ratio stability, and a high level of resolution. As such DCC bridges, for example, have replaced resistance ratio instruments such as the Wenner and Kelvin bridges for resistance measurements. Typically, comparator implementations provide accuracies within the range of 0.1 ppm to 1.0 ppm.
However, with the continued drive for improved accuracy in calibration, standards, and measurements on circuits and components operating at hundreds of kiloVolts, thousands of Amps, with resistances into Gigaohms accuracies of parts per million is being replaced by parts per billion. For example, Guildline Instruments offers a range of DCC bridges starting at 100 parts per billion over the range 0.001Ω to 100 kΩ to extended performance DCC bridges at fifty (50) parts per billion for measurements up to 100 MΩ and currents to 3000 A or forty (40) parts per billion for measurements up to 1 GΩ and 1000V. Other DCC bridges operate at even lower errors of fifteen (15) parts per billion.
At these levels the inventors have identified that a variety of factors, such as interruptions to measurement cycles, e.g. the bridge is unplugged mid-measurement, and manufacturing variations in the current comparator toroids, can generate residual flux within the toroids sufficient to generate offsets within the range of ten (10) to twenty (20) parts per billion. Accordingly, the inventors have established a current comparator design and control approach to address these offsets.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.